I'm new to Korean, and it's possible I might have heard it wrong. But for BTS - I Need U and Forvo (pronunciation website), I hear "chakku". It doesn't seem to matter what precedes or follows the word, but it's always a "ch" sound instead of an expected "j" sound.

2 Answers
2

Pronunciation?

What you've found is right: unvoiced at the beginning of a word, otherwise voiced.

Then why “j” instead of “ch?” ― Romanization consistency for Koreans

And as to the title, that's because of the rigid romanization that wants a consonant to have the same romanization across words and sentences, irrespective of the actual pronunciation. You can find out the reason why in the writing below. The current romanization standard was standardized on July 7 of 2000.

Written in 2001 by Seh-joong Kim, the head of Department of Language Research, National Institute of Korean Language.

Another drawback of the previous standard was that it didn't make much sense to Koreans. To them, the voiced and unvoiced sounds are just allophones, so having them have different romanizations is not a good way to go, considering what really “romanization” is for. It is a nonsense to them that “부부” should be written “pubu,” and a Korean romanization standard that Koreans themselves don't understand was a wrong decision to begin with.

But still, it sucks. Every Korean romanization method does. “어금니” goes “Eogeumni” and “내의령” goes “Naeuiryeong.” Supposing I have zero knowledge of the Korean language, how the heck am I supposed to read that out? You can see how messed up this is here, compared to Japanese or some other cutout languages for romanization. I can say this: Korean and Roman letters don't go well. Really, if you're to learn Korean, you need to throw that shit out. I'm not exaggerating.

Initial Consonant
An initial consonant is any consonant at the beginning of a word. Initial consonants (especially at the beginning of sentences and phrases) are usually unvoiced. For example, the ㅈ in the word 저 ("I") is typically voiceless, especially as the first word of a sentence. That makes it sound more like "ch" than "j" to an English speaker. The consonants that follow this rule are ㄱ, ㄷ, ㅈ, and ㅂ. Thus, an initial ㄱ sounds more like "k" than "g", an initial ㄷ sounds more like "t" than "d" and an initial ㅈ sounds more like "ch" than "j". However, an initial ㅂ sounds more like "B" than "p":